Thick film capacitors usually comprise a lower and upper electrode with a dielectric layer between. During the thick film manufacturing process, a number of variables will affect the ultimate value of a capacitor. Variations in materials, alignment and thickness of printed components all contribute to variations in the final component. Typically, a printed capacitor placed at a critical point in a circuit will be trimmed in the circuit until the capacitor achieves a desired circuit performance.
Capacitors in the past have been adjusted by abrasively trimming the upper layers until the area of the upper layer provides a capacitor of the desired value. This has required leaving the newly printed capacitor exposed to environmental influences until the part has been trimmed and encapsulated. There are a number of prior art devices and methods of making them. A typical prior method teaches that a tail portion on the upper layer of the thick film capacitor is to be abraded to arrive at a desired capacitance. The capacitor is then encapsulated, but the component is exposed during the entire trimming process.
As technology has progressed, thick film circuits have increased in density, and conventional dual electrode capacitors have required a disproportionate amount of substrate area. Furthermore, it is now desirable to design a capacitor which can withstand laser or optical trimming. Conventional thick film capacitors which are trimmed by abrasive methods can be fabricated with dielectric materials which have dielectric constants as high as E.sub.r =20. It has been shown that materials which can withstand laser trimming have dielectric constants only as high as E.sub.r =10. In order to design a capacitor which can withstand laser trimming, it would necessarily be twice as large as a capacitor designed to be abraded. The present invention overcomes these problems by providing a thick film capacitor which is linearly trimmable over its entire range. The present invention incorporates a unique layout in which one or more of the lower layers of the electrodes are compared to segmented fingerlike electrodes partially exposed to enable electrical isolation through trimming. This arrangement provides a Fifty Percent (50%) increase in a capacitance per unit area while incorporating materials which can withstand laser trimming. Furthermore, the capacitor is encapsulated prior to trimming and, therefore, provides a component which is more stable during the manufacturing process.